This disclosure relates to protection of bearing contact surfaces in heat transfer systems such as compressors and pumps.
Heat transfer systems such as refrigeration systems typically include one or more lubricants mixed in a halocarbon heat transfer fluid or refrigerant. The lubricant can help protect bearing contact surfaces such as in compressor rotor bearings from wear. Various oils have been used as refrigerant lubricants, including mineral oils, polyol esters, polyalkylene glycols, alkylbenzenes, polyalpha olefins, or polyvinyl ethers. A typical refrigeration system can operate with levels of lubricant at key locations in flowing refrigerant. The presence of an oil reservoir, typically with more than a kilogram of oil will cause overall content of the heat transfer fluid to exceed 1 percent by weight, based on the total weight of the heat transfer fluid. The concentration will be relatively low in the condenser (e.g., 50 ppm to 500 ppm). At other locations, the concentrations will be higher. For example the oil sump may contain 60 percent oil or more. This oil-rich portion is used to lubricate bearings. Thus, flow to the bearings will typically be well over 50 percent oil. At one or more locations in the system, strainers, stills, or other means may be used to withdraw oil and return it to a reservoir.
However, the use of lubricants in heat transfer fluids can present a number of challenges. For example, the thermodynamic properties of the lubricant will typically be different than those of the refrigerant, which leads to unwanted accumulation of the lubricant in portions of the system where it is not needed for lubrication, such as in the evaporator, instead of at the compressor where it is needed. This can necessitate maintaining higher overall levels of lubricant in order to maintain the desired lubricant content in the oil sump, which tends to adversely affect heat transfer efficiency for the system. Oil separators and oil transfer sumps conduits are often used to remove lubricant from unwanted locations and direct it to compressor. However, such measures can add to system cost, complexity, and maintenance and repair requirements.
The challenges presented by the lubricants used in refrigeration systems have led to interest in oil-free refrigerant systems. Such systems have tended to focus on bearing design and material selection to try and reduce or eliminate the need for lubricants to be added to refrigerant. For example, Lapp et al., WO2013085969 discloses silicon nitride ceramic balls rolling on nitrogen alloy steel races. Although such approaches can improve bearing surface durability, there are still concerns about long term reliability due to wear of the bearing surfaces due to friction. In addition, there has been concern about the breakdown of refrigerants in oil-free systems and the formation of refrigerant breakdown products, including acids leading to corrosion and polymeric deposits that block refrigerant flow through system components. Kujak et al., WO 2014/117014 A2 discloses the use of corrosion inhibitor or stabilizer additives to address the impact of these refrigerant breakdown products, but still requires the presence of a lubricant.